1. Field of the Invention
The invention pertains to digital imaging systems, and more particularly to a digital imaging system with reduced flicker caused by fluorescent lighting.
2. Description of the Related Art
Digital imagers have become commonplace in the last decade. For both video cameras and still cameras, semiconductor devices are commonly used to capture an image on a pixel-by-pixel basis and electronically process that image. Such devices as charge coupled devices (CCDs) and CMOS digital imagers have resulted in low cost video cameras, still cameras, and more recently cameras for coupling to computer systems for video conferencing and other image capture.
One problem pertaining to imaging systems generally and to digital imagers in particular is that of flicker. Flicker can arise from many sources, but in capturing digital video flicker especially results from a relationship between some periodic phenomena and the frame rate of the camera. Digital video cameras capture images on a frame-by-frame basis, typically at a predetermined frame rate. A common frame rate in the United States and in the computer industry is 30 Hz. But when such a frame rate is used in Europe, for example, flicker can result from fluorescent lighting systems employing the standard 50 Hz alternating current power. A 50 Hz lighting system yields periodic peaks of intensity at a rate of 100 Hz, or once every 10 milliseconds. Digital imaging systems often pick up "beats" associated with this 100 Hz intensity peak being captured at a 30 Hz rate. Beats can also arise from very slight differences in fundamental frequencies such as between 69.47 Hz video and 60 Hz lighting.
A number of solutions have been employed to eliminate these "beats." These include filtering systems that filter out beat frequency, phase locking systems that attempt to lock on to the 100 Hz intensity peaks and synchronize frame capture, and a variety of other techniques.